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Author Topic: Radio Frequency Interference (RFI) in Mics  (Read 21031 times)

klaus

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Radio Frequency Interference (RFI) in Mics
« on: March 29, 2011, 01:57:55 pm »

Originally Posted:Mon, 13 September 2004

Radio Frequency Interference (RFI) in microphones can be a pest- it comes and goes, then reappears as soon as one has forgotten about it, to damage a recording in progress.
RFI can be successfully and permanently eliminated in almost all cases.
I would like to share which remedies I have found are least impacting on the mic's sound while still taking care of the problem.
 
RFI comes in two audible forms: distinct radio station signals mixed in with the mic's audio, and noise hash, whose presence is much harder to hear and properly identify as RFI, because it appears as featureless background white noise.
 
How does a microphone pick up RFI?
RFI is often a result of inappropriate cable terminations, insufficient cable shielding methods or materials and/or certain designs of mic circuitry. More rarely, RFI may be triggered by a very strong radio transmitter close by (which in turn exacerbates some of the triggers mentioned above)
 
Think of the microphone, including its cable, as an antenna: unlike other audio gear, the mic, as the first link in a recording chain, is open ended on one side (towards the head of the mic) and electrically connected only on the other with an amplification device which is nicely suitable to transmit what's been captured from this antenna.
 
Certain configurations of the mic's processor (f. ex. Nuvistors, discreet op amps, electronically balanced, rather than truly balanced, mic outputs) are often more prone to RFI than others.
 
How to get rid of RFI?

I use a methodical approach to curing RFI, based on my experience of what needs to be addressed first, second, and third, regardless of other potential variables. If I do not address the problem in this order, I may miss a cure and waste time.
 

1. Cable Terminations
 Inspect the mic cable's connector terminations. I mean visually inspect, not just assume, their correctness. Make sure that the following terminations are present:
 
- On phantom powered mics (and other mics with XLR three-pin connectors):
 Pin #1 and the ground lug of each connector's housing must be connected together, on both cable ends!
 
Pin #1 of the XLRs must then be connected to the cable's ground wire. If the cable does not have a dedicated ground wire, the cable's shield must be connected to pin #1. If the cable has both, ground wire and shield (Gotham, Neumann), connect both to pin #1 of the XLRs.
 
Beware of certain connectors (some Neutriks comes to mind) which do not have a separate ground lug termination point for the connector's housing. Do not use these connectors on mic cables, as the connector shells cannot be grounded.
 
When you are done, do the following test to confirm the correct connections:
An ohm meter's leads connected to both connector housings should read zero or a few single digit ohms. If it reads open, go back and inspect what's wrong, or you will get RFI!
 
On tube mics:
Connect ground wire and cable shield together to the pin of the connector that is dedicated to ground and install a wire from ground to make contact with the connector's housing. This connection is often conveniently made at the cable strain relief, where a clamp is screwed into threads of the connector housing.
 
Again: install this ground/shield scheme on both sides of the cable, and when done, perform the ohm meter test, as described above.
 
This method of termination is always applicable for microphone cables, except for rare multi-connector hook ups (I won't get into this here.) Please do not mix up correct RFI terminations with what you have learned about terminations to prevent ground loops- there are no ground loops with mics and their cables, as they are single ended.
 The terminations described above also happen to be the official Neumann recommendation.

Gunnar Hellquist adds:
 
Quote:

The microphone should be electrically insulated from the holder and hence from the stand.

If not, the mic is no longer ground loop-poof, as contaminating electricity, picked up from somewhere else, may flow through the conductive mic stand to the mic)
 

2. Cable material
 I will not discuss advantages of certain cable brands, or their effect on sound. (I recommend and endorse, without payment, a specific microphone cable- Gotham, Switzerland)
I will share though, that in several independent cable tests for RFI susceptibility, one of them in an official AES paper, cables with double Reussen layer shields were provably more resistant to RFI than any other method of cable shielding.

Reussen layer cable shields are nothing else but multi strands of copper wire twisted in a cork screw fashion around the cable conductors. A second layer is then twisted in an opposite direction over the first. This is visibly different from braided shields or foil shields.

To my knowledge, only Neumann, Berlin, and Gotham, Switzerland (whose Reussen cables are now made by Belden in Germany) use this type of shielding.
 
My experience has been that in nasty cases of RFI, this type of cable, in combination with proper terminations cured the problem when correct terminations alone did not.
 
3. Chokes
A choke is a wire coiled in such a way that certain frequencies of audio when it passes through the coil, are suppressed.
Typically these choke coils, which in mics are very small and look like resistors, are inserted in series at the balanced outputs of the mic.
 
This method of RFI suppression really works well- like a sledge hammer on mosquitoes: The deterioration of sound from choke coils ranges from subtle to unbearable, depending on the value of the coil chosen.

Typical values of coils factory installed in mics range between 40 and 100ÁH (choke coils are rated in ÁH.)
I have found that anything above 25 micro-Henry is not tolerable, as the coil effect pretty much messes up the phase of the midrange of the mic's audio.
 

4. Ferrite beads
Schoeps pioneered the use of tiny hollow magnet beads inserted over the wires of the impedance converter inputs, i.e. the ferrite bead is inserted over the FET's gate wire or the tube's grid wire. The magnetic effect of these strategically placed beads interferes with the RF component of audio (don't ask me how it works)

Greg Youngman adds a link for more info on ferrite beads:
 
Quote:

http://www.amidoncorp.com/aai_ferritesshieldingbeads.htm

He further comments:

Quote:

...(microphone) cables can act like antennas if not properly terminated or shielded. A cold solder joint on a connector can act like a diode and invite unwanted RF... A certain cable length is "tuned" or "resonant" and susceptible to a certain frequency of RF energy if not properly terminated.

I have always been able to kill RFI with method #1,2, and 3, and have not had the need to experiment with #4. Therefore, I have no personal experience with this method of RF suppression and welcome explanations and personal experiences regarding the use of ferrite beads in mics.
------------
 
I would also recommend to protect against other external interferences (fluorescent bulb generators, eletro-magnetic motors, dimmers, and other stray fields) with the above described methods- they all belong in a category of external fields triggering audible disturbances when entering a microphone and its cable.
 
To sum up: RFI can be successfully suppressed in 100% of all cases where it occurs. (Even without the use of choke coils, my success rate is about 90%.) Curing RFI takes a methodical, step by step approach, where nothing is assumed but everything is inspected, then tested.
 
P.S.: I will leave this thread open for a while, so others can contribute as well. In time, I will condense these contributions into the main text, then lock it.
 
Kind regards,
 
Logged
Klaus Heyne
German Masterworks
www.GermanMasterworks.com
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